Fix a problem with NSTask blocking the main thread.

NSTask seems to block the app while the shell command is running.
Added an option to `shell` to run in background and ignore the output.
More options can be added to `options` parameter as needed.

Example:

bind "S", ["cmd", "alt", "ctrl"], -> shell '/usr/bin/say', [api.selectedText()], null, null, {"donotwait": true}